Repositories and Units of Work - which is responsible for what?

Question

Over the past week or so I have been reading a lot of articles and tutorials regarding the repository pattern. A lot of the articles closely tie the repository pattern to the unit of work pattern. In these articles, I usually find code similar to this:

interface IUnitOfWork<TEntity>
{
    void RegisterNew(TEntity entity);
    void RegisterDirty(TEntity entity);
    void RegisterDeleted(TEntity entity);

    void Commit();
    void Rollback();
}

interface IRepository<TKey, TEntity>
{
    TEntity FindById(TKey id);
    IEnumerable<TEntity> FindAll();

    void Add(TEntity entity);
    void Update(TEntity entity);
    void Delete(TEntity entity);
}

class Repository : IRepository<int, string>
{
    public Repository(IUnitOfWork<string> context)
    {
        this.context = context;
    }

    private IUnitOfWork<string> context;

    public void Add(string entity)
    {
        context.RegisterNew(entity);
    }

    public void Update(string entity)
    {
        context.RegisterDirty(entity);
    }

    public void Delete(string entity)
    {
        context.RegisterDeleted(entity);
    }

    /* Entity retrieval methods */
}

Am I understanding correctly that the unit of work object is meant to handle the addition, update, or deletion of any object in the underlying data store (in my case, a directory service which I communicate with via LDAP)? If that's true, shouldn't it handle the retrieval of any objects as well? Why is that not part of the suggested UoW interface?

Answer 1

A Repository is in charge of the data - getting it, updating and the other CRUD operations, providing persistence ignorance.

A Unit Of Work (uow), as Marin Fowler says:

Maintains a list of objects affected by a business transaction and coordinates the writing out of changes and the resolution of concurrency problems.

The uow will coordinate multiple operations on objects - it may or may not use repositories in order to persist these changes.